This project is a continuation of our earlier studies on the primary structure and postribosomal amino acid modifications of cardiac and skeletal muscle myosin. We propose here to investigate the subunit structure, charge properties, contents of methylated and phosphorylated amino acids, partial amino acid sequence, and ATP-ase activity of uterus smooth muscle myosin. Subsequently, comparative studies will be carried out on the various aspects of skeletal versus uterus muscle myosin as well as of myosins in non-pregnant and pregnant uterus. It is expected that the results will help to elucidate a) structural similarities or differences in skeletal and uterus muscle myosins, b) a possible change in the pattern of myosin gene expression in uterus muscle during pregnancy, c) the structural basis for the variation in uterus contractility during pregnancy, and d) the functional importance of amino acid sidechain modifications in myosin. The information will be applicable to the mechanism and regulation of uterus contraction, more specifically to the problems of onset of labor and premature labor. Furthermore, the data of uterin smooth muscle myosin will be relevant to the research on myosin of vascular smooth muscle from the wall of blood vessels, where the lack of experimental material delays progress.